Use of pre-coated mat for preparing gypsum board

ABSTRACT

A gypsum board which comprises a set gypsum core sandwiched between and faced with fibrous mats, wherein a free surface of one of said mats is pre-coated with a combination of a mineral pigment, optionally an inorganic adhesive binder and an organic binder, preferably a hydrophobic, UV resistant polymer latex adhesive binder applied to said surface as an aqueous coating composition, said aqueous coating composition upon drying and setting providing a pre-coated mat satisfying certain morphology requirements.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. Ser. No.10/798,891 filed Mar. 12, 2004, the entire contents of which areincorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to the use of a pre-coated fiber mat for makingfiber mat-faced gypsum board, for example, a gypsum board faced with aglass fiber mat. The invention more particularly relates to the fibermat-faced gypsum board that is prepared with the pre-coated fiber mat.The coating on the pre-coated mat comprises a dried aqueous mixture of amineral pigment or filler, an organic binder, preferably comprised of ahydrophobic, UV-resistant polymer latex adhesive; and, optionally asecond binder comprised of an inorganic adhesive, wherein the driedcoating satisfies certain coating morphology requirements.

The present invention is based on the discovery that in order to obtaina gypsum board with certain desired physical strength characteristics,the pre-coated mat used to prepare the gypsum board product must satisfya particular set of structural properties and the process for making theboard must produce the proper interface between the gypsum core and thepre-coated mat.

BACKGROUND OF THE INVENTION

Panels of gypsum wallboard which comprise a core of set gypsumsandwiched between two sheets of facing paper have long been used asstructural members in the fabrication of buildings where the panels areused to form the partitions or walls of rooms, elevator shafts,stairwells, ceilings and the like.

In efforts to mitigate or overcome problems associated with the use ofpaper-faced gypsum wallboard in applications where moisture exposure isexpected to occur, the prior art has approached the problem in variousways over the years.

One approach to the problem has been to treat the paper comprising thefacing of the wallboard with a water-resistant material sometimesreferred to as a water-repellant. Polyethylene emulsion is an example ofa material that is used to treat paper facing to impart water-resistantcharacteristics. Such treatment is designed to deter delamination of themulti-ply paper facing by reducing the tendency of the paper to absorbwater which is a chief cause of delamination and to deter water frompenetrating through the paper to the gypsum and destroying the bondbetween the paper-facing and gypsum core.

Another approach to the problem has involved incorporating into theformulation from which the gypsum core is made a material that functionsto impart improved water-resistant properties to the set gypsum coreitself. Such an additive tends to reduce the water-absorbing tendency ofthe core and decrease the solubility characteristics of the set gypsum.Wax-asphalt emulsions and wax emulsions are examples of such additives.

Although improvements had been realized by the provision of gypsumwallboard prepared in accordance with these teachings, furtherimprovements were still possible. Experience showed that even with suchconstructions the paper facing delaminated and the gypsum core erodedthrough the degrading action of moisture. The problem was particularlyaggravated by warm water acting upon a gypsum core that includes eithera wax emulsion or a wax-asphalt emulsion, commonly used water-resistantcore additives. While cores containing such materials have relativelygood water-resistant characteristics in the presence of water at roomtemperature, such characteristics start to fall off at temperatures inexcess of 70° F. and tend to disappear in the presence of water having atemperature of about 100° F. or higher.

In another commercially successful approach, a structural panelcomprising a set gypsum core sandwiched between two porous fibrous matsis provided, see U.S. Pat. No. 4,647,496. The preferred form of mat isdescribed as a non-woven glass fiber mat formed from fiberglassfilaments oriented in random pattern and bound together with a resinbinder. Such panels differ from conventional gypsum wallboard in thatthe fibrous mat is substituted for paper as the facing material(s) ofthe gypsum core. In such constructions, the set gypsum from the coreextends at least part-way into the fibrous mat facer to form an integralattachment/bond between the gypsum and the mat. In this construction theflow of gypsum into the mat during preparation of the board isunimpeded. Since the mat is completely porous, the gypsum flows freelyinto the pores (interstices) of the mat forming a strong bond with themat.

The strength of the bond between the gypsum core and the fibrous facingis a critical factor in the usefulness of the panel in applications thatrequire the attachment of tiles, insulating material (foam insulation)or other exterior finishing materials onto the fibrous surface. If thebond does not have a sufficient strength, the board is not strong enoughto satisfy its supporting function. In that instance, the face sheetdelaminates from the gypsum core. Experience has shown that the bondmust have a tensile strength of at least about 16 psi, measured by thebond strength test described in detail below, in order to adequatelyprovide the required supporting function.

Extensive outdoor testing has shown that glass mat-faced,water-resistant gypsum board of the type described in the aforementioned'496 patent has much better weathering characteristics, includingwater-resistant characteristics, than water-resistant gypsum boardcovered with water-resistant paper facings.

In a more recent improvement of this technology, as described in U.S.Pat. No. 5,397,631, the fibrous mat-faced gypsum board following initialboard preparation is coated with a latex polymer. The coating, which isapplied onto the fibrous facing of the gypsum panel after the board hasbeen prepared, acts as both a liquid and vapor barrier (vapor permeanceof about 1.2 perms (ASTM E-96)).

The coating is formed from an aqueous coating composition comprisingfrom about 15 to about 35 wt. % of resin solids, about 20 to about 65wt. % of filler, and about 15 to about 45 wt. % of water, applied toobtain a solids loading of at least about 50 lbs. per 1000 sq. such asabout 110 lbs. per 1000 sq. ft. A preferred resin for use according tothis patent is a latex polymer that has been sold by Unocal ChemicalsDivision of Unocal Corporation under the mark 76 RES 1018. The resin isa styrene-acrylic copolymer that has a relatively low film-formingtemperature.

In accordance with the teachings of the '631 patent, the aqueouscoatings composition formed from the resin is not applied to the fibrousmat-facing of the gypsum board until after the board has been prepared.The post-applied coating is dried effectively at oven temperatureswithin the range of about 300° to 400° F. If desired, a coalescing agentcan be used to lower the film-forming temperature of the resin. Sincethe bond between the fibrous mat and board core is formed during boardpreparation, the post-applied coating does not affect the strength ofthat bond and the board is able to satisfy it supporting function justas do the boards of the '496 patent.

More recently, a coated fibrous mat-faced gypsum board of surprisinglyeffective moisture resistance, having a predominantly inorganic coatingon the mat, was developed, see U.S. Published Application 2002/0155282,which is incorporated herein by reference. The mat used to prepare thegypsum board described in this published application was pre-coated witha predominately inorganic coating containing a minor amount of anorganic binder and the pre-coated fibrous mat then was used as one ofthe facers in the manufacture of a gypsum board. Surprisingly, thecoating on the pre-coated mat had sufficient porosity to allow watervapor to permeate through the mat during manufacture of the board, butprovided the board with unexpectedly effective moisture resistance.

Using a pre-coated mat to manufacture the board significantly simplifiedthe manufacture of the board. The coating was comprised of a mineralpigment (pigmented filler material), an inorganic binder and a latexpolymer binder. In particular, the coating comprised a dried (or cured)aqueous mixture of a mineral pigment; a first binder of a polymer latex(organic) adhesive and, a second binder of an inorganic adhesive. On adry weight basis, the first polymer latex binder comprised no more thanabout 5.0% by weight of the coating, and the second inorganic bindercomprised at least about 0.5% by weight, of the total weight of thecoating.

Because the board described in this published application was designedto be used in situations where the surface of the board is not requiredto support the attachment of tiles, insulating material or otherconstruction materials, e.g., exterior finishing materials, the strengthof the bond between the pre-coated mat and the gypsum core was oflimited importance. Indeed, the board of this published application hasnot been used in such situations and the bond between the pre-coated matand the gypsum core in the disclosed board would not inherently have thenecessary tensile strength for such uses.

A manufacturer of a pre-coated mat of the type used in making the gypsumboard of the U.S. Published Application 2002/0155282 recently publishedits own application, U.S. Published Application 2003/0134079 directed toproducing an improved coated glass mat using substantially the samecoating formulation. The '079 published application is directedprincipally to an improved procedure for making a coated mat purportedlyhaving improved tensile (tear) properties. The '079 application suggestsusing the pre-coated mat for making a gypsum panel product andspeculates about mat characteristics desirable for that specificapplication.

In particular, the '079 published application speculates that there is afine balance between the portion of coated mat and portion of uncoated,exposed glass fibers in the mat needed to make the pre-coated matsuitable for making a gypsum panel product. As described, if too muchglass mat is exposed such that the gypsum cannot cover it entirely, theapplication speculates that the resultant board is unacceptable.Conversely, if not enough glass fibers are left exposed, the '079application indicates that the resulting board also is not acceptable.In both cases, the '079 application speculates the board may fail aflexural stress test.

The '079 application goes on to suggest that a proper balance isobtained when the coating penetrates into the glass mat substrate to adepth of from 25 percent to 75 percent of the thickness of the coatedglass mat. The '079 application does not address the situation where thegypsum board in intended to be used in circumstances where it isrequired to support a substrate such as tiles, insulating material orother exterior finishing materials.

The present inventors have determined that the coating parameterspostulated by the applicants of the '079 application are not suitablefor producing a gypsum panel, using current commercial manufacturingtechniques, that can be used in situations where tiles, insulatingmaterial or other exterior finishing materials must be supported by thepanel. In particular, unless the extent of coating penetration in thepre-coated mat is controlled within a much smaller window and theprocess conditions for subsequently making the gypsum board are properlyobserved, the resulting bond between the pre-coated glass mat and thegypsum core does not have the required tensile strength for supportingtiles, insulating material or other construction materials, such asexterior finishing materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features, and advantages of the invention will be apparentfrom the following more detailed description of certain embodiments ofthe invention and as illustrated in the accompanying drawings. Thedrawings are highly schematic and are not necessarily to scale, emphasisinstead being placed upon illustrating the features of the invention.

FIG. 1, shows a highly schematic view of an apparatus for making thegypsum board of the present invention and the board being assembledthereon.

FIG. 2 is a highly schematic cross-sectional view of a section of apre-coated mat suitable for making a gypsum board according to thepresent invention.

FIG. 3 is a highly schematic cross-sectional view of a section of agypsum board according to the present invention illustrating theinterface of the pre-coated mat and gypsum core.

FIG. 4 is a color photomicrograph showing the interface between a coatedfiber mat and the gypsum core in a gypsum board.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a gypsum board of the present invention (10) can bemanufactured by enmeshing a set gypsum board core formed from a gypsumslurry (23) with at least one, and preferably two pre-coated fiber mats,(14) and (16). Preferably, both mats are predominately glass fiber mats.The surface of at least one of the mats (and preferably both of themats) has been pre-coated with a dried (heat cured) coating (indicatedby the numeral (15) in the figures) of an aqueous coating compositioncontaining a combination (e.g., a mixture) of a mineral pigment orfiller; an organic binder, preferably a UV resistant polymer latexadhesive binder having a suitable level of hydrophobicity (ahydrophobic, UV resistant polymer latex) and, optionally a second binderof an inorganic adhesive. By “pre-coated” is meant that the mat has adried, adherent coating of what was originally an aqueous coatingcomposition, as hereinafter defined in more detail, applied to itssurface before the mat is used to make the gypsum board of the presentinvention.

The fiber mat(s) comprise(s) a fiber material that is capable of forminga strong bond with the set gypsum comprising the core of the gypsumboard through a mechanical-like interlocking between the interstices(pores) of the fiber mat and portions of the gypsum core filling thoseinterstices. Examples of such fiber materials include (i) a mineral-typematerial such as glass fibers, (ii) synthetic resin fibers and (iii)mixtures or blends thereof. Based on economic considerations, glassfiber mats are preferred for making the pre-coated mat.

The fiber mats can comprise continuous or discrete strands or fibers andcan be woven or nonwoven in form. Nonwoven mats such as made fromchopped strands and continuous strands can be used satisfactorily andare less costly than woven materials. The strands of such mats typicallyare bonded together to form a unitary structure by a suitable adhesive.

Non-woven glass mat suitable for use in the present invention can beprepared by a wet-laid process, which is carried out on what can beviewed as modified papermaking machinery. Descriptions of the wet-laidprocess for making glass mats may be found in a number of U.S. patents,including U.S. Pat. Nos. 2,906,660, 3,012,929, 3,050,427, 3,103,461,3,228,825, 3,760,458, 3,766,003, 3,838,995, 3,905,067, 4,112,174,4,681,802 and 4,810,576, all of which are incorporated herein byreference.

In general, the wet-laid process for making non-woven glass fiber matscomprises first forming an aqueous slurry of short-length glass fibers(referred to in the art as “white water”) under agitation in a mixingtank, then feeding the slurry onto a moving screen on which the fibersenmesh themselves into a freshly prepared wet glass fiber mat, whileexcess water is separated from the mat of fibers. Most, if not all ofthe fibers used to make the mat have a length somewhere between aboutone-quarter (¼) to about one (1) inch, and more usually from aboutone-quarter (¼) to about three-quarter (¾) inch and have diameters inthe range of 10 to 16 microns.

Machines such as wire cylinders, Fourdrinier machines, Stevens Former,Roto Former, Inver Former and Venti Former machines and the like can beused to form the wet-laid mat. In such equipment, a head box depositsthe dilute slurry onto a moving wire screen. Suction or vacuum removesthe water resulting in the wet-laid mat. Usually, an upwardly inclinedwire having several linear feet of very dilute stock lay-down, followedby several linear feet of high vacuum water removal, is used. This isfollowed by a binder applicator, such as a “curtain coater,” thatapplies the glass fiber binder and then by an oven that removes excesswater and cures (dries) the adhesive to form a coherent fiber matstructure.

Fiber mats made in this way have an open pore structure with a highpercentage of fiber-fiber interstitial space.

Fiber mats are commonly made in widths anywhere from 144 to 240 inchesand collected in roll form for storage and handling, e.g.,transportation. For use in making gypsum boards, rolls of fiber matshaving a width between about 23 and 57 inches typically are desired andcan be obtained in those widths directly from the mat manufacturingprocess or can be cut from rolls of greater widths.

With reference to FIG. 2, a suitable fiber mat for use in the presentinvention can range in thickness (30), for example, from about 20 toabout 45 mils, with a mat thickness (30) of about 25 mils to about 35mils generally being preferred.

Fiber mats meeting these requirements are known and are commerciallyavailable. In many cases the thickness (30) of a specific fiber mat willbe variable across its structure, possibly both in the machine (i.e.,manufacturing) direction, i.e., along its length or major dimension, andacross its width or minor dimension (cross-machine direction).

In addition to the fiber diameter, fiber length and mat thickness, theweight per unit surface area at which the non-woven glass fiber mat isprepared also influences the properties of the mat important for makinga pre-coated mat suitable for use in making a gypsum wallboard by thecontinuous gypsum board production method. Broadly the mat should beprepared at a weight per unit surface area (hereinafter also referred toas the basis weight) of from about 1 lb./100 ft.² to no greater thanabout 3 lb./100 ft². More specifically, it is preferred that the fibermat, especially a non-woven glass fiber mat, be prepared at a basisweight of at least about 1.4 lb./100 ft.² but no greater than about 2.7lb./100 ft² (measured at an uncompressed mat thickness (30) in the rangeof about 20 mils to about 45 mils), with a nominal thickness of about 33mils being the most typical.

A suitable fiber mat for making the pre-coated mat used in the presentinvention is a fiberglass mat comprising chopped, nonwoven, fiberglassfilaments oriented in a random pattern and bound together with a resinbinder, usually a urea-formaldehyde resin adhesive. Fiber glass mats ofthis type are commercially available, for example, such as those whichhave been sold under the trademark DURA-GLASS® by Manville BuildingMaterials Corporation and those which have been sold by Elk Corporationas BUR or shingle mat.

One glass fiber mat, which is particularly useful for preparing apre-coated mat for making a gypsum board of the present invention, isnominally 0.030 to 0.033 inch (30 mils to 33 mils) (0.76 to 0.83 mm)thick and incorporates glass fibers about 13 to 16 microns in diameter.Another glass fiber mat nominally 20 mils thick, which includes glassfibers about 10 microns in diameter, is also suitable for use in thepresent invention. Manville mat JM 7594 is one suitable fiber mat andhas a thickness generally within the range of 0.030 to 0.042 inch (30 to42 mils).

A coated glass mat suitable for use in preparing the gypsum board of thepresent invention has a coating (15) applied from an aqueous coatingformulation to the thus-prepared fiber mat, which coating hassubstantially uniformly penetrated the open-pore structure of the mat.In accordance with the present invention, the coating is applied suchthat it penetrates into and envelopes the fibers of the glass fiber matto a depth of from about 30 percent to about 50 percent of the thickness(30) of the coated glass fiber mat. With reference to FIG. 2, thecoating depth of mat penetration (31) is from about 30 percent to about50 percent of the total mat thickness (30), preferably from about 35 to50 percent. With this degree of coating penetration, the un-coated matportion (32) comprises about 50 percent to about 70 percent (morepreferably 50 to 65 percent) of the total mat thickness (30). Thisportion (32) is available for penetration by gypsum slurry duringpreparation of a gypsum board.

The applied coating has a morphology characterized by a film coveringand to a certain extent bridging individual fibers in the mataccompanied by a distribution of micropores (not shown) through the filmcoating (15). The distribution of micropores in the coating (15) issufficient to allow air-flow there through during board manufacture thusallowing for the gypsum slurry to fill the fiber-fiber interstices ofthe non-coated side (32) of the mat during the initial stages in thepreparation of the gypsum board and to allow a sufficient curing anddrying of the gypsum core during the completion of the manufacture ofthe board. In other words, the coating is sufficiently porous to permitthe gypsum slurry to displace air through the mat during the first phaseof board preparation and to permit water in the aqueous gypsum slurryfrom which the gypsum core is made to evaporate in its vaporous statethere through during the final manufacturing steps of the board. Themorphology and composition of the coating nonetheless acts as a barrierto the influx of moisture during subsequent use of the board.

The extent of penetration and the related morphology of the coating onthe mat should be relatively uniform across the entire surface of themat. As used in the specification and claims, the phrase “relativelyuniform” means that the extent of coating penetration and other coatingmorphological parameters, such as coating density and porosity(microporosity), as hereinafter defined, should remain within a desiredrange over 50 percent of the surface area of the mat, preferably over 75percent of the surface of the mat and most preferably over 90 percent ofthe surface of the mat. Preferably, the extent of penetration and othercoating morphological parameters also do not vary by more than 50percent over the surface area of the mat, preferably do not vary by morethan 30 percent and most preferably do not vary by more than 20 percentacross the mat surface area.

The coating composition, which is applied to one, free surface of theabove-described fiber mat for making the pre-coated mat for use in thepresent invention, comprises an aqueous combination of predominately amineral pigment or filler; an organic adhesive binder, preferably ahydrophobic, UV resistant polymer latex adhesive; and, optionally asecond inorganic binder of an inorganic adhesive. On a dry weight basisof the two essential components (100%), the organic binder comprises atleast about 1% and no more than about 17% by weight, with the balancebeing the inorganic, mineral pigment or filler. Optionally a secondinorganic binder preferably comprising at least about 0.5% by weight, ofthe total weight of the dried (cured) coating, but no more than about20% by weight of the coating also can be present. The weight ratio ofthe mineral pigment or filler to the polymer latex adhesive (organic)binder can be in excess of 15:1 and in some cases can be in excess of20:1, but usually is at least about 5:1.

Suitable coating compositions for making the pre-coated mat useful inthe present invention thus may contain, on a dry weight basis of thethree noted components (100%), about 75 to 99 percent mineral pigment orfiller, more usually about 83 to 95 percent mineral pigment or filler,about 0 to 20 percent inorganic adhesive, more usually about 0 to 10percent and about 1 to 17 percent organic adhesive binder, preferably ahydrophobic, UV resistant polymer latex adhesive, more usually about 1to 12 percent.

A mineral pigment or filler comprises the major component of the coatingcomposition. Examples of mineral pigments suitable for making coatedmats useful in the present invention include, but are not limited to,ground limestone (calcium carbonate), clay, sand, mica, talc, gypsum(calcium sulfate dihydrate), aluminum trihydrate (ATH), antimony oxide,or a combination of any two or more of these substances.

The mineral pigment is provided in a particulate form. To be aneffective mineral pigment for making a coated mat for use in thisinvention, the pigment should have a particle size such that at leastabout 95% by weight of the pigment particles pass through a 100 meshwire screen, with about 75% of the particles (by number) being greaterthat 5 μm. Preferably, the pigment has most of, if not all of, the fineparticles removed, especially particles less than 1 μm. It has beenobserved that the presence of an excess amount of fine particles in thecoating composition negatively impacts the porosity (microporosity ormicropore structure) of the pre-coated mat. A preferred mineral pigmentis a limestone having a number average particle size of about 40 μm.Such materials are collectively and individually referred to in thealternative as mineral pigments or as “fillers” throughout the remainderof this application.

The second essential constituent, the organic adhesive binder includessuch materials as styrene-butadiene-rubber (SBR),styrene-butadiene-styrene (SBS), ethylene-vinyl-chloride (EVCl),poly-vinylidene-chloride (PVdC), modified poly-vinyl-chloride (PVC),poly-vinyl-alcohol (PVOH), ethylene-vinyl-agitate (EVA), andpoly-vinyl-acetate (PVA). Preferably, the organic binder is ahydrophobic, UV resistant, polymer latex binder adhesive, includingpolymers and copolymers containing units of acrylic acid, methacrylicacid (together referred to as (meth)acrylic acids)), their esters(referred to together as ((meth)acrylates) or acrylonitrile.

Ordinarily these latexes of UV resistant polymers are made by emulsionpolymerization of ethylenically unsaturated monomers. Such monomers mayinclude (meth)acrylic acid, 2-hydroxyethyl(meth)acrylate,2-hydroxypropyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate,methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate,isopropyl(meth)acrylate, butyl(meth)acrylate, amyl(meth)acrylate,isobutyl(meth)acrylate, t-butyl(meth)acrylate, pentyl(meth)acrylate,isoamyl(meth)acrylate, hexyl(meth)acrylate, heptyl(meth)acrylate,octyl(meth)acrylate, isooctyl(meth)acrylate, 2-ethylhexyl(meth)acrylate,nonyl(meth)acrylate, decyl(meth)acrylate, isodecyl(meth)acrylate,undecyl(meth)acrylate, dodecyl(meth)acrylate, lauryl(meth)acrylate,octadecyl(meth)acrylate, stearyl(meth)acrylate,tetrahydrofurfuryl(meth)acrylate, butoxyethyl(meth)acrylate,ethoxydiethylene glycol(meth)acrylate, benzyl(meth)acrylate,cyclohexyl(meth)acrylate, phenoxyethyl(meth)acrylate, polyethyleneglycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate,methoxyethylene glycol(meth)acrylate, ethoxyethoxyethyl(meth)acrylate,methoxypolyethylene glycol(meth)acrylate, methoxypolypropyleneglycol(meth)acrylate, dicyclopentadiene(meth)acrylate,dicyclopentanyl(meth)acrylate, tricyclodecanyl(meth)acrylate,isobornyl(meth)acrylate, and bornyl(meth)acrylate. Other monomers whichcan be co-polymerized with the (meth)acrylic monomers, generally in aminor amount, include styrene, diacetone(meth)acrylamide,isobutoxymethyl(meth)acryl amide, N-vinylpyrrolidone, N-vinylcaprolactam, N,N-dimethyl(meth)acrylamide, t-octyl(meth)acrylamide,N,N-diethyl(meth)acrylamide, N,N′-dimethyl-aminopropyl(meth)acrylamide,(meth)acryloylmorphorine; vinyl ethers such as hydroxybutyl vinyl ether,lauryl vinyl ether, cetyl vinyl ether, and 2-ethylhexyl vinyl ether;maleic acid esters; fumaric acid esters; and similar compounds.

Suitable hydrophobic, UV resistant polymer latex binder adhesives arepreferably based on a (meth)acrylate polymer latex, wherein the(meth)acrylate polymer is a lower alkyl ester, such as a methyl, ethylor butyl ester, of acrylic and/or methacrylic acids, and copolymers ofsuch esters with minor amounts of other ethylenically unsaturatedcopolymerizable monomers (such as styrene) which are known to the art tobe suitable in the preparation of UV resistant (meth)acrylic polymerlatexes. Another suitable co-monomer is vinyl acetate, which may be usedas a co-monomer with, for instance, butyl acrylate in a ratio of 70/30or smaller of the vinyl acetate to the butyl acrylate.

One particularly useful hydrophobic, UV resistant polymer latex binderadhesive is NeoCar® Acrylic 820. NeoCar® Acrylic 820 is an ultra-smallparticle size, hydrophobic latex available from Dow Chemical Company andis apparently made by copolymerizing a highly branched vinyl ester withan acrylate. Other suitable hydrophobic, UV resistant polymer latexbinder adhesives include Glascol® C37 and Glascol® C44 available fromCiba Specialties Chemical Corporation; Rhoplex® AC-1034 available fromRohm & Haas and UCAR® 626 available from Dow Chemical Company.

As used throughout this specification and in the claims, the termshydrophobic, hydrophobicity and the like are intended to embrace UVresistant polymers, which yield a three (3) minute Cobb value of belowabout 1.5 grams for the pre-coated fibrous mat. UV resistant polymersthat exhibit a three (3) minute Cobb value of below about 0.5 grams areparticularly preferred for making the pre-coated fibrous mat. The threeminute Cobb value of a resin is determined by a simple procedure whichis similar to TAPPI procedure T441. According to the procedure, a coatedtest mat is prepared by coating a standard glass mat with an aqueouscoating formulation and dried at 230° F. (110° C.) for 20 minutes. Thecoating formulation is prepared by combining 70 parts by weightlimestone having an average particle size of about 40 μm (GFP 102available from Global Stone Filler Products or equivalent) with 17 partsby weight (dry solids basis) of the latex resin and blending thoroughlyfor 30 seconds. The aqueous formulation is applied to the mat using asimple knife applicator to obtain a dry basis weight of between about 22grams of coating per sq. ft. on the glass mat (standard glass mat—JohnsManville mat 7594 or equivalent).

A 5.25 inches by 5.25 inches square sample of the coated mat isobtained, weighed and then secured in a 100 cm² Cobb ring. One hundredmilliliters of warm (120° F. (49° C.)) water is poured into the ring asrapidly as possible and retained there for 2 minutes and 50 seconds.Then, the water is poured from the ring as quickly as possible (withoutcontacting any other portion of the sample). At the three minute mark, aCouch roller is used with a sheet of blotting paper (rolled forward andbackwards once) to remove excess moisture from the sample. The samplethen is weighed and the increase in weight is recorded. The test isrepeated once and the average of the two weight increase values isconsidered the three minute Cobb value for that sample. Again, UVresistant latex resins exhibiting a three minute Cobb value of 1.5 gmsor below in this test are preferred for use in the present invention.

An optional component of the coating composition is an inorganicadhesive binder. Examples of inorganic adhesive binders which can beused in combination with the polymer adhesive latex binder(s) in thecoating composition for making a pre-coated fibrous mat useful in thisinvention include, but are not limited to the following: calcium oxide,calcium silicate, calcium sulfate (anhydrous or hemi-hydrate), magnesiumoxychloride, magnesium oxysulfate, and other complex inorganic bindersof some Group IIA elements (alkaline earth metals), as well as aluminumhydroxide.

One example of a complex inorganic binder is common Portland cement,which is a mixture of various calcium-aluminum silicates. However,Portland cement cures by hydration, which can create a coating mixturewith a short shelf life. Also, both the oxychloride and the oxysulfateof magnesium are complex inorganic binders, which cure by hydration.Coating formulations made with such inorganic adhesive binders must beused quickly or a tank containing the aqueous coating composition couldset up in a short period of time.

The oxychlorides or oxysulfates of magnesium, aluminum hydroxide, andcalcium silicate are only very slightly soluble in water, and are usefuloptional inorganic adhesive binders of this invention. Inorganicadhesive binders, which are quickly soluble in water, such as sodiumsilicate, may not be usable in coatings expected to be exposed to hotand/or high humid ambient conditions for long periods. One preferredinorganic adhesive binder for making a coated mat useful in thisinvention is quicklime (CaO). Quicklime does not hydrate in a coatingmix, but cures by slowly converting to limestone, using carbon dioxidefrom the air. Quicklime is not (or only very sparingly) soluble inwater.

Inorganic pigment or filler materials inherently containing somenaturally occurring inorganic adhesive binder also can be used to make,and often are preferred for making the coated mat used in the presentinvention. Examples of such fillers, some listed with the naturallyoccurring binder, include (but are not limited to) the following:limestone containing quicklime (CaO), clay containing calcium silicate,sand containing calcium silicate, aluminum trihydrate containingaluminum hydroxide, cementitious fly ash and magnesium oxide containingeither the sulfate or chloride of magnesium, or both. Depending on itslevel of hydration, gypsum can be both a mineral pigment and aninorganic adhesive binder, but it is only slightly soluble in water, andthe solid form is crystalline making it brittle and weak as a binder. Asa result, gypsum is not generally preferred for use as the optionalinorganic adhesive binder.

Fillers, which inherently include an inorganic adhesive binder as aconstituent and which cure by hydration, also advantageously act asflame suppressants. As examples, aluminum trihydrate (ATH), calciumsulfate (gypsum), and the oxychloride and oxysulfate of magnesium allcarry molecules of water bound into their molecular structure. Thiswater, referred to either as water of crystallization or water ofhydration, is released upon sufficient heating, actually suppressingflames.

Low cost inorganic mineral pigments and fillers such with the propertiesof those described in the preceding paragraph, thus, may provide three(3) important contributions to the coating mixture: filler; a binder;and, a fire suppressor.

In addition to the two essential components and the one optionalcomponent, the aqueous coating composition will also include water.Water is provided in an amount sufficient to provide desired rheologicalproperties (e.g., viscosity) to the aqueous coating composition. Desiredrhelogical properties are those which, given the chosen form ofapplication of the aqueous coating composition to the fiber mat, providefor the retention of the coating formulation on the surface of thefibers of the fiber mat and within the interstices of the fiber mat to adepth between 30 and 50 percent of the thickness of the fiber mat from afree surface of the mat and the formation of the desired coatingmorphology (microporosity). The aqueous coating composition will usuallyalso include other optional ingredients such as colorants (e.g.,pigments), thickeners or theological control agents, defoamers,dispersants and preservatives. When used, the aggregate amount suchother ingredients in the coating composition is typically in the rangeof 0.1 to 5% and generally is not more than about 2% of the principalthree above-noted components. Generally, the aqueous coating formulationwill have a viscosity between about 8,000 and 18,000 cps.

Any suitable method for applying an aqueous coating composition to thefiber mat substrate can be used for making the pre-coated mat, such asroller coating, curtain coating, knife coating, brush coating, spraycoating and the like, including combinations thereof, provided that thedepth of coating is limited to within the range of 30 to 50 percent ofthe mat thickness.

Following application of the aqueous coating composition to the mat, thecomposition is dried (cured), typically in a drying oven, to form thepre-coated mat. The mat is dried at a temperature and for a timesufficient to remove the water from the coating composition and coalescethe organic polymer adhesive to form an adherent coating, withoutdegrading the coating or the mat. Suitable temperatures and times willbe influenced greatly by the equipment being used and can be obtained bythose skilled in the art using routine experimentation.

As will be appreciated by those skilled in the art, the extent to whichthe aqueous coating formulation penetrates into the fiber mat isinfluenced on the one hand by the properties of the fiber mat itself andon the other hand by the various coating parameters such as the aqueouscoating viscosity, the time between applying the coating and drying(curing) the coating (e.g., the speed of the coating line), the rate ofapplication of the coating, the method by which the coating is appliedand the like. Varying these parameters to obtain the desired extent ofpenetration in any particular mat requires no more than routineexperimentation to those skilled in the art.

The pre-coated mat made in accordance with these teachings iseffectively liquid impermeable, but does allow air and water vapor topass through, sufficiently to be used in making a gypsum board by thecontinuous gypsum board manufacturing process.

The pre-coated fiber mat used in the preparing the gypsum board of thepresent invention can be prepared by applying an aqueous coatingcomposition containing the noted solid constituents to a fiber mat, asidentified above, in an amount on a dry weight basis equivalent to atleast about 30 lbs., more usually between about 30 and 100 lbs., per1000 sq. ft. of mat. Normally, the dry coating is present in an amountequivalent to at least about 30 to 50 lbs. depending upon the thicknessof the glass fiber mat.

Previous work has shown that certain UV resistant latex resins of thedesired hydrophobicity are useful for making a pre-coated fiber matuseful in ultimately making an improved gypsum panel particularly usefulfor exterior applications (pending U.S. application Ser. No.10/417,344). The earlier work also indicated that mats coated withcompositions made using such hydrophobic, UV resistant latex resins(polymers), within certain compositional constraints, were sufficientlyporous to be used for making a gypsum board using conventionalmanufacturing procedures. Unfortunately, this prior work did notrecognize the relationship between pre-coated mat properties, such ascoating morphology and coating depth, and the strength of thefacer-board bond which determines the ability of the board to adequatelysupport tiles, insulating material or other construction materials, suchas exterior finishing materials from the pre-coated mat surface of theboard.

Thus, the organic binder must provide, in combination with the mineralpigment or filler and the optional inorganic adhesive, when dried toform the adherent coating a desired level of porosity (microporosity ormicropore structure) at the noted usage levels and coating density. Thecoating porosity is determined by an easily performed test, which isdescribed in detail below. Organic binders failing to meet the porosity(microporosity) requirement are not suitable for use in the coatingcomposition of the present invention for making the pre-coated fibrousmat.

The test for porosity (microporosity) is a modification of the procedureof TAPPI T460, Gurley method for measuring the air resistance of paper.In this procedure, a sample of the coated mat (approximately 2 inches by5 inches) is clamped between the 1 in² orifice plates of a GurleyDensometer, Model 4110. The inner cylinder is released and allowed todescend under only its own weight (i.e. by gravity alone) and theelapsed time (measured in seconds) between the instant the innercylinder enters the outer cylinder of the apparatus until the 100 mlmark on the inner cylinder reaches (enters) the outer cylinder isrecorded. The test then is repeated with the sample facing (oriented) inthe opposite direction. The porosity, reported in seconds, comprises theaverage of the two replicates for each sample. A suitable resin exhibitsa porosity of less than about 45 seconds, preferably less than about 20seconds. At porosities of higher than about 45 seconds, the coatedmat-gypsum core interface is at a much higher risk of delamination(i.e., blister formation) as the water vapors seek a path to escapeduring curing of the board. As discussed below, too low of a porosityalso interferes with air flow through the coating during boardpreparation as the gypsum slurry seeks to penetrate into and through thenon-coated side of the mat and leads to formation of an unsatisfactoryinterface between the gypsum core and the coating. Preferably, theporosity is also more than about 2 seconds and usually more than about 5seconds, so as to minimize bleedthrough of gypsum during boardmanufacture.

Gypsum boards made with the pre-coated fibrous mat of the presentinvention have superior weathering characteristics, and accordingly, canbe used effectively for indefinite periods of time as a stable substratein outdoor applications involving extended exposure to the sun,prolonged water contact and high humidity.

The gypsum core of the gypsum board of the present invention isbasically of the type used in those gypsum structural products, whichare known as gypsum wallboard, dry wall, gypsum board, gypsum lath andgypsum sheathing. The core of such a product is formed by mixing waterwith powdered anhydrous calcium sulfate or calcium sulfate hemi-hydrate(CaSO₄.½H₂O), also known as calcined gypsum to form an aqueous gypsumslurry, and thereafter allowing the slurry mixture to hydrate or setinto calcium sulfate dihydrate (CaSO₄.2H₂O), a relatively hard material.

The composition from which the set gypsum core of the structural panelis made can include a variety of optional additives, including, forexample, those included conventionally in gypsum wallboard. Examples ofsuch additives include set accelerators, set retarders, foaming agents,reinforcing fibers (e.g., glass fibers), and dispersing agents.

A preferred gypsum core of the present invention also includes one ormore additives, which improve the water-resistant properties of thecore. In particular, the coated fibrous mat-faced gypsum board for usein the present invention preferably comprises a gypsum core, which haswater-resistant properties. The preferred means for impartingwater-resistant properties to the gypsum core is to include in thegypsum composition from which the core is made one or more additives,which improve the ability of the set gypsum composition to resist beingdegraded by water, for example, to resist dissolution.

Examples of materials which have been reported as being effective forimproving the water-resistant properties of gypsum products are:poly(vinyl alcohol), with or without a minor amount of poly(vinylacetate); metallic resinates; wax or asphalt or mixtures thereof,usually supplied as an emulsion; a mixture of wax and/or asphalt andalso cornflower and potassium permanganate; water insolublethermoplastic organic materials such as petroleum and natural asphalt,coal tar, and thermoplastic synthetic resins such as poly(vinylacetate), poly(vinyl chloride) and a copolymer of vinyl acetate andvinyl chloride and acrylic resins; a mixture of metal rosin soap, awater soluble alkaline earth metal salt, and residual fuel oil; amixture of petroleum wax in the form of an emulsion and either residualfuel oil, pine tar or coal tar; a mixture comprising residual fuel oiland rosin; aromatic isocyanates and diisocyanates; organopolysiloxanes,for example, of the type referred to in U.S. Pat. Nos. 3,455,710;3,623,895; 4,136,687; 4,447,498; and 4,643,771; siliconates, such asavailable from Dow Corning as Dow Corning 772; a wax emulsion and awax-asphalt emulsion each with or without such materials as potassiumsulfate, alkali and alkaline earth aluminates, and Portland cement; awax-asphalt emulsion prepared by adding to a blend of molten wax andasphalt an oil-soluble, water-dispersing emulsifying agent, and admixingthe aforementioned with a solution of case in which contains, as adispersing agent, an alkali sulfonate of a polyarylmethylenecondensation product. Mixtures of these additives can also be employed.

A mixture of materials, namely, one or more of poly(vinyl alcohol),siliconates, wax emulsion and wax-asphalt emulsion of the aforementionedtypes, for example, also can be used to improve the water resistance ofgypsum products, such as described in aforementioned U.S. Pat. No.3,935,021, which is incorporated herein in its entirety.

Typically, the core of fibrous mat-faced gypsum board has a density ofabout 40 to about 55 lbs. per cu. ft., more usually about 46 to about 50lbs per cu. ft. Of course, cores having both higher and lower densitiescan be used in particular applications if desired. The manufacture ofcores of predetermined densities can be accomplished by using knowntechniques, for example, by introducing an appropriate amount of foam(soap) into the aqueous gypsum slurry from which the core is formed orby molding.

The pre-coated fiber mat-faced gypsum board can be made efficiently, asis well known, by forming an aqueous gypsum slurry which contains excesswater and placing the gypsum slurry on a horizontally oriented movingweb of the pre-coated fiber mat, with the coated mat surface orientedaway from the deposited gypsum slurry. In a preferred embodiment,another moving web of fiber mat, which optionally can also be thepre-coated fiber mat, but for example also can be a glass mat, a matmade from a blend of glass and synthetic fibers, or a pre-treated mat,is then placed on the upper free surface of the aqueous gypsum slurry.Aided by heating, excess water evaporates through the pre-coated mat asthe calcined gypsum hydrates and sets.

In order for the pre-coated mat to be most useful in making the coatedmat-faced gypsum board of the present invention, it is preferred thatthe coated mat be rolled into rolls of continuous sheet. As a result,the coated mat cannot be so stiff and brittle that it will break uponbending. To accomplish this objective, it appears that the inorganicadhesive binder content of the mat coating, when present in aformulation, should not exceed about 20% by weight of the total dryweight of the coating, and usually is less than about 10%. As noted,such rolls are typically employed in widths between 23 and 57 inches.

The moisture tolerant structural panels of this invention comprising apre-coated fibrous mat-faced gypsum board can be made utilizing anexisting, manufacturing line for gypsum wallboard as illustrated inFIG. 1. In conventional fashion, dry ingredients from which the gypsumcore is formed can be pre-mixed and then fed to a mixer of the typecommonly referred to as a pin mixer (20). Water and other liquidconstituents, such as soap, used in making the core are metered into thepin mixer where they are combined with the desired dry ingredients toform an aqueous gypsum slurry. Foam (soap) is generally added to theslurry, such as in the pin mixer or in a separate mixer, to control thedensity of the resulting core.

The gypsum slurry is dispersed through one or more outlets from themixer onto a moving sheet (fibrous mat) (16), which is indefinite inlength and is fed from a roll thereof onto a forming table (21) andadvanced by conveyor (22). The sheet (16) includes a coating (15) (seealso FIG. 2) on what constitutes the bottom surface of the sheet as fedto the forming table. As described above, the coating comprises a driedaqueous mixture of a mineral pigment; an organic binder preferablycomprising a hydrophobic, UV-resistant polymer latex adhesive; and,optionally a second binder comprised of an inorganic adhesive.

In order to obtain a bond of a sufficient strength between the uncoatedside of the mat and the gypsum core, so as to permit the board tosupport tiles, insulating material or other construction materials, suchas exterior finishing materials from the pre-coated mat surface of theboard, it is critical that the gypsum slurry flow into and through theinterstices of the mat so as to meet with the internal (inner-most)border of the coating of the coated mat. This construction isschematically shown in FIG. 3. The flow of gypsum slurry into theuncoated side of the pre-coated mat and through to the internal surfaceof the coating must be so complete as to cause this interface to besubstantially free of entrapped air. In other words, the interfacialcontact between the gypsum core penetrating the uncoated surface of themat and the coating must be substantially free of voids.

As shown in FIG. 3, the gypsum board (10) of this invention hasbasically three regions on the coated mat side of the board: (31), (32)and (33), schematically indicated by different cross-hatching in thefigure. Region (31) is the outermost board face and contains the coatedportion of the fiber mat facer having the mat coating (15). Region (33)is the gypsum core and contains principally set gypsum and other commongypsum additives. This region is free of the fiber mat facer. Finally,region (32) contains that portion of the fiber mat facer that was freeof the coating composition (uncoated mat) but has become imbibed withthe gypsum slurry during board preparation and has subsequently set toform set gypsum. As shown, there is substantially complete contactbetween the mat coating (15) and the set gypsum in the imbibed region(32). Together with the mechanical interlocking with the fiber mat, thiscontinuous interface between the set gypsum core and the mat coating iswhat contributes to the development of a bond of a sufficient strengthbetween the mat and the gypsum core, so as to permit the coated matsurface of the board to support tiles, insulating material or otherconstruction materials, such as exterior finishing materials from thepre-coated mat surface of the board.

The microporosity of the coating on the fiber mat thus permits thegypsum slurry to displace air through the mat during the initial stagein the preparation of the gypsum board. If the coating on the mat lacksthe necessary microporosity, the air in the interstices of the fiber matis unable to escape and a proper interfacial contact between thepenetrating gypsum core and the mat coating is not formed.

FIG. 4 illustrates poor gypsum penetration through the interstices ofthe fiber mat to the inner mat surface of the coating. As shown in FIG.4, the coating on the glass mat facer has penetrated for the most partto a depth of from about 0.15 to in some instances as much as 0.27 mm (6to about 11 mils) in a glass mat having a thickness of about 0.76 to0.82 mm (30 to 32 mils). This constitutes an average penetration depthof about 25 to 27 percent of the thickness of the mat. As a consequenceof the insufficient coating penetration and formation of the propercoating morphology, the gypsum slurry was unable to penetrate (flow)into the uncoated side of the mat sufficiently to form continuousinterfacial contact between the coating and the set gypsum core. As aresult one can see voids in the vicinity of the coating-core interfaceas well as portions of the glass fiber mat that are not imbibed with setgypsum.

However, the coating on the mat cannot be so porous as to allow gypsumto freely penetrate through the coating and cause gypsum bleed thru ontothe face of the product. A preferred coating porosity (microporosity) isbetween 5 and 20 seconds.

One convenient way to ensure an adequate formation of a substantiallycontinuous interface between the gypsum core and the internal surface ofa properly coated fiber mat is to discharged one stream of gypsum slurrythrough outlet (17) to provide a relatively thin layer of aqueouscalcined gypsum slurry (18) on the non-coated surface of sheet 16. Thethin layer of gypsum slurry (18) is somewhat more dense than the aqueousslurry of gypsum that is used to form the main portion of the core ofthe gypsum board (main core slurry discharged through outlet (19) toforma gypsum slurry layer (23)). The penetration of this higher densityslurry into the interstices of the fiber mat and the elimination of anyentrapped air at the gypsum-coating interface can be added by a roller(not shown). The proper penetration of this thin layer of gypsum intothe interstices of the fiber mat helps to form the strong bond betweenthe lower density portion of the core and the pre-coated mat facer.Typically, the slurry used to form the thin layer (18) is about 18-20%more dense than the density of the slurry (23) used to form the mainportion of the core. A portion of this denser slurry also is often usedto form hard edges of the gypsum board as well.

In this illustrative embodiment, pre-coated mat (16) thus forms one ofthe facing sheets of the gypsum board. As noted above, the pre-coatedmat is fed with the coated side facing away from the gypsum slurry. Theslurry (preferably denser slurry (18)) penetrates sufficiently into andthrough the thickness of the pre-coated glass mat, on the back-side, ornon-coated side of the mat, to form a bond between the subsequently setgypsum, the fibrous mat and the dried adherent coating previouslyapplied to the fibrous mat. Thus, on setting, a strong adherent bond isformed between the set gypsum and the pre-coated fibrous mat. In partbecause of the coating on the surface of the mat, the slurry does notpenetrate completely through the mat.

As is common practice in the manufacture of conventional paper-facedgypsum board, the two opposite edge portions of the sheet (16) areprogressively flexed upwardly from the mean plane thereof and thenturned inwardly at the margins as to provide coverings for the edges ofthe resulting board. One of the benefits of the pre-coated mat used inconnection with the present invention is that it has shown sufficientflexibility to form acceptable board edges.

In a preferred embodiment of the invention, another fiber mat (14), alsosupplied in roll form and made in the same fashion as mat (16), is takenfrom the roll and fed around a roller (7) onto the top of the gypsumslurry (23) to form facing sheet (9), thereby sandwiching the gypsumslurry (core) between the two moving pre-coated glass fiber sheets. Thefiber mats (16) and (14) thus form facings on the set gypsum core thatis formed from the gypsum slurry to produce the gypsum board withopposite fiber mat facers. Alternatively, mat (14) may be uncoated andin that case preferably is made from a blend of glass fibers andpolyester fibers as described in U.S. Pat. No. 5,883,024. One source ofsuch a mat is Johns Manville mat 8802. The mat also could be a standardglass fiber mat. The mat (14) is applied to the top of the gypsumslurry. Thus, as above, a strong bond also is formed between this matand the gypsum core as previously described.

Conventional shaping rolls and edge guiding devices (not shown)typically are used to shape and maintain the edges of the compositeuntil the gypsum has set sufficiently to retain its shape. After the(top) fiber mat (14) is applied, the “sandwich” of fiber mats and gypsumslurry can be pressed to the desired thickness between plates (notshown). Alternatively, the fiber mats and slurry can be pressed to thedesired thickness with rollers or in another manner. The continuoussandwich of slurry and applied facing materials then is carried byconveyor(s) (22). Slurry (23) sets as it is carried along.

Although improvements can be realized by the use of a gypsum core whichhas but one of its surfaces faced with the pre-coated fiber mat asdescribed herein, it is believed that, for some applications, it may beadvantageous to manufacture board having both surfaces faced with thepre-coated fiber mat. The weight of the board (nominal ½″ thickness)usually should not exceed about 2500 lbs. per 1000 sq. ft. Typically,the board will weigh at least about 1600 lbs. per 1000 sq. ft.

The ability of the pre-coated fiber mat used in the present invention topass water vapor therethrough is an important feature of the presentinvention and is such that the drying characteristics of the board arenot substantially altered relative to a board faced with conventionalpaper facing. This means that industrial drying conditions typicallyused in continuous gypsum board manufacture also can be used in themanufacture of pre-coated mat-faced board of the present invention.Exemplary drying conditions include dryer (oven) temperatures of about200° to about 700° F., with drying times of about 30 to about 60minutes, at line speeds of about 70 to about 400 linear feet per minute.

The extent of drying also needs careful control to insure that theresulting board can adequately be used for supporting tiles, insulatingmaterial (e.g., foam insulation) or other exterior finishing materialson the surface of the pre-coated mat. Too much drying can result insurface calcination of the gypsum and a reduction in bond strengthbetween the pre-coated mat and the gypsum core. In this regard,applicants have determined that the amount of combined water must be atleast 17% (fully hydrated gypsum has a level of combined water of about21%) in the vicinity of the facer-core interface.

The level of calcination (combined water) in the mat facer-gypsum coreinterface is measured using an O'Haus Moisture Balance Model No. MB 200and the following procedure. Representative samples (12 inches by 8inches) of the board product are collected. The coated glass mat faceris peeled from the sample and the board is placed on a surface suitablefor collecting the scrapings generated in the next step. A two footsquare piece of paper should be suitable. The entire exposed surface iscarefully scraped off of the board to a depth of about 10 mils (0.254mm) to remove the surface interface. At least 10 g of scrapings arecollected, screened through a 50 mesh screen to remove any glass fibersand then tested for Combined Water using the O'Haus Moisture Balance setat 205° C. and 25 minutes. The change in weight provides the percent ofcombined water.

Boards of the present invention can be used effectively in many outdoorapplications in addition to those previously mentioned. The board isparticularly suitable for applications where tiles, insulating materialor other construction materials, such as exterior finishing materialsare supported by the gypsum board. For example, the gypsum board can beused in applications of the type where conventional gypsum sheathing isapplied as a support surface for overlying materials such as woodsiding, stucco, synthetic stucco, aluminum, brick, including thin brick,outdoor tile, stone aggregate and marble. The aforementioned finishingmaterials can be used advantageously in a manner such that they areadhered directly to the coated board. The board of the present inventionhas the necessary tensile strength to satisfactorily support suchmaterials. The board of the present invention can also be used also as acomponent of exterior insulating systems, commercial roof deck systems,and exterior curtain walls. In addition, the board can be usedeffectively in applications not generally involving the use ofpaper-faced gypsum board. Examples of such applications include wallsassociated with saunas, swimming pools, gang showers, or as a substrateor component of a secondary weather barrier.

The examples that follow are illustrative, but are not to be limiting ofthe invention.

Example 1

Pre-coated fibrous mats have been prepared by first preparing thefollowing coating composition.

Ingredients Amounts, wt. % Aqueous acrylic latex (45% solids) 18.7(NeoCar ® 820) Limestone 65.3 (GFP 102 from Global Stone FillerProducts) Ethyl hydroxyethyk cellulose thickener/stabilizer 0.04Bermocoll 230FQ Acrylate thickeners 0.19, 0.19 Paragum 501, 109 Colorant0.47 Englehard W 1241 Ammonia 0.37 Added water 14.74

The aqueous coating composition has been applied to Johns Manville 7594fiberglass mat at application rates of about 30 grams per square foot(about 66 pounds per 1000 square feet). The wet coating composition wasdried. The dried basis weight of the coating was about 22 grams persquare foot (about 48 pounds per 1000 square feet).

Example 2

Pre-coated fiberglass mat obtained in the manner of Example 1 was usedto prepare gypsum board panels. The pre-coated mat had a thickness ofabout 35 mils and a porosity of about 10 seconds.

Continuous length boards were made from a gypsum slurry containing about55% percent by weight of gypsum hemi-hydrate and the pre-coated mats ona conventional wallboard machine. The slurry was deposited on onecontinuous sheet of the coated mat, which was advanced at a rate ofabout 185 linear feet per minute, sufficient to form ⅝th inch thickboards, while a continuous sheet of Johns Manville 8802 fibrous mat wasdeposited onto the opposite surface of the gypsum slurry. Drying of thegypsum boards was accelerated by heating the composite structures in anoven at about 600° F. for about thirty minutes and until the boards arealmost dry and then at about 250° F. for about fifteen minutes untilthey are dried completely. The densities of the coated mat-faced boardswere determined to be about 50 lb. per cu. ft.

Coated mat-faced gypsum boards made in accordance with the presentinvention are capable of resisting for indefinite periods of time attackby water, both in indoor and outdoor applications, and to offersignificantly enhanced fire resistance. In summary, it can be said thatthe improved gypsum-based product of the present invention haswater-tolerant properties which are at least equal to or better thanprior art products, and that this is achieved in a product that isobtained in a product that is as light as and more economical to makethan prior art products.

Example 3

Selected gypsum boards made with pre-coated mats were tested to assessthe bond strength between the pre-coated mat facer and the gypsum core.

In order to support tiles, insulating material or other constructionmaterials, such as exterior finishing materials from the pre-coated matsurface of the board, the bond between the gypsum core and the mat facermust have a sufficient strength. In particular, it has been determinedthat the tensile strength of that bond should be at least 16 psi whentested in accordance with the following procedure.

A 5.5 inch square sample for testing is prepared from a gypsum board. Anexpanded polystyrene board (EPS) then is adhered to the gypsum boardsample. An EPS board, nominal 2 pound, one inch thick (typical of EPSinsulation), cut into a 3.5 inch square, is suitable. A suitableadhesive is a 1:1 mixture of Type I Portland cement and Dryvit Primusadhesive mixed with water to a suitable consistency for applying it toEPS board. This adhesive requires about two weeks to cure.

Thereafter, a wood block (1.5 inches thick) is glued to the free side ofthe EPS board using a fast curing (e.g., five minute) epoxy. The woodblock is sized to match the EPS board (3.5 inches square) and isprovided with two slots on opposite edges to permit its attachment tothe tensile test equipment. Using a Satec Universal Testing Machine(Model UTC60HVL-1437), the slots should be one-quarter inch wide andthree-quarter inch deep. The slots should be cut on the edges of theblock that are perpendicular to the grain of the wood. The sample shouldbe ready for testing in approximately one hour.

The specimen is attached to the test equipment and the machine is set tooperate in the 0 to 600 pound range at a speed of one inch per minute.During the test, the gypsum board is pulled from the EPS board as thewood block restrains the sample. The machine is run until the maximumload has been reached and the specimen has failed. The final result fora specific board is preferably the average of three samples.

The Table below lists the tensile test results of several samples thatwere prepared with pre-coated mats. The mats had different degrees ofcoating penetration. The boards were prepared on commercial board linesusing standard production techniques. Samples 1, 2 6 and 7 were madeusing a coating formulation of the type described in U.S. Publishedapplication 2002/0155282. Sample 1 was made using a Venrotex fiber glassmat having a basis weight of 2.1 lb per 100 square feet. Samples 6 and 7used a glass fiber mat having a basis weight of 1.4 pound per 100 squarefeet of the type described in U.S. Published application 2002/0155282.Sample 2 used a Johns-Manville fiber glass mat. Sample 3, 4 and 5 weremade using a Johns-Manville mat and a coating formulation as describe inExamples 1 and 2. The gypsum boards of sample 1, 2 and 3 were ⅝ inchthick, while the remaining samples were ½ thick boards.

Coating Percent Penetration Coating Avg. Board Sample No. (mils)Penetration Tensile (psi) 1 15 43.6 19.5 2 11.6 37.7 17.1 3 12.9 34.418.6 4 11.8 29.9 18.6 5 6.9 19.7 13   6 14.1 64  0* 7 13.9 58  0**Reject board severe blistering after exiting the dryer.

It will be understood that while the invention has been described inconjunction with specific embodiments thereof, the foregoing descriptionand examples are intended to illustrate, but not limit the scope of theinvention. Unless otherwise specifically indicated, all percentages areby weight. Throughout the specification and in the claims the term“about” is intended to encompass + or −5%.

Other aspects, advantages and modifications will be apparent to thoseskilled in the art to which the invention pertains, and these aspectsand modifications are within the scope of the invention, which islimited only by the appended claims.

1. A gypsum board comprising: a gypsum core having a main portion and asecond portion, the second portion being more dense than the mainportion; a fiber mat comprising a first side and second side oppositethe first side, the second side is bonded to the second portion of thegypsum core such that a tensile strength of the bond is at least 16pounds per square inch; and a coating penetrating the fiber mat from thefirst side into the fiber mat to a depth of about 30 percent to about 50percent of the thickness of the fiber mat, the coating comprises amineral pigment and an organic binder.
 2. The gypsum board of claim 1,wherein the second portion is about 18 to 20% more dense than the firstportion.
 3. The gypsum board of claim 1, wherein the mineral pigment hasa number average particle size of about 40 microns.
 4. The gypsum boardof claim 1, wherein the organic binder comprises a hydrophobic, UVresistant polymer latex binder made by copolymerizing a vinyl ester withan acrylate.
 5. The gypsum board according to claim 4, wherein the vinylester is vinyl acetate and where the acrylate is butyl acrylate.
 6. Thegypsum board according to claim 5, wherein a ratio of the vinyl acetateto the butyl acrylate is 70/30 or smaller.
 7. The gypsum board accordingto claim 1, wherein the gypsum core in the region of the bond has atleast 17 percent combined water.
 8. A gypsum board comprising: a gypsumcore having a main portion and a second portion, the second portionbeing more dense than the main portion; a first fiber mat comprising afirst side and second side opposite the first side, the second side isbonded to the second portion of the gypsum core such that a tensilestrength of the bond is at least 16 pounds per square inch; a firstcoating penetrating the first fiber mat from the first side into thefiber mat to a depth of about 30 percent to about 50 percent of thethickness of the fiber mat, a second fiber mat comprising a first sideand second side opposite the first side, the second side is bonded tothe main portion of the gypsum core; a second coating penetrating thesecond fiber mat from the first side into the fiber mat to a depth ofabout 30 percent to about 50 percent of the thickness of the fiber mat;wherein both the first fiber mat and the second fiber mat each compriseglass fibers nominally about 10 to 16 microns in diameter and aboutone-quarter (¼) to about one (1) inch in length, the first fiber mat inthe absence of coating has a basis weight of 1 to 3 pounds per 100square feet.
 9. The gypsum board of claim 8, wherein the first coatingand the second coating each comprise an organic binder comprising ahydrophobic, UV resistant polymer latex binder made by copolymerizing avinyl ester with an acrylate.
 10. The gypsum board according to claim 9,wherein the vinyl ester is vinyl acetate and where the acrylate is butylacrylate.
 11. The gypsum board according to claim 10, wherein a ratio ofthe vinyl acetate to the butyl acrylate is 70/30 or smaller.